A vs. The
Even though we do have a separate section on articles, it may still be tricky to determine whether a sentence needs the definite or indefinite article.
Remember that the choice always depends on the context. Use a when talking about things in general. Use the when talking about something specific.
Example 1
— Do you know a person named Joe?
— Have you met him before? Are you looking for someone specific?
— No, any Joe would be ok. I am just wondering. I have never met anyone named Joe before.
Example 2
— Do you know the person named Joe?
— Have you met him before? Are you looking for someone specific?
— Yes, I met him last Friday, and I am still trying to find him.